Echolocation
Certain animals are known to possess the ability to echolocate. This is known as the production of sound waves that bounce back to the source as information echo signals when they reach the target. This ability of animals to echolocate is similar to the mechanism of the sonar. Therefore, a better understanding of the principles of echolocation especially in bats and dolphins can lead to the development of better and improved technology involving the sonar and echolocation. Moreover, a basic knowledge of the rudimentary principles that govern the mechanism of echolocation may lead to a better understanding of how animals react to their environment. Echolocation in animals is both simple and complex and although the principles may vary depending on the species, the evolutionary origin and the general mechanism are very similar in all echolocating species.
Echolocation in Certain Animals
Certain animals use a remarkable system of navigation known as echolocation. This particular navigation system is mainly used by these animals for hunting prey as well as for finding themselves in the dark. So far, the animals that are known to possess the ability to echolocate include bats, shrews, dolphins, whales and swiftlets.
Bats. Bats are actually the first animals found to possess echolocation. Bats use this ability to help them find their way and their prey in the dark considering the fact that bats are nocturnal creatures. The process is mainly done through the production of sound that results from the air that rushes from the lungs and passes the vocal cords. The resulting vibrations of the vocal cords then form a sound wave.
The echolocation of bats is mainly used for catching prey especially small insects. Through echolocation, the insect is usually detected, located, and intercepted in flight within about half a second (Griffin et al.). Typically a bat would average around 10 mosquitoes or 14 fruit flies per minute during a period of several minutes. (Griffin et al.)
The bats that possess the ability to echolocate include the suborder Microchiroptera within the mammalian order Chiroptera with over 800 species (Kruse). The echolocation of these bats is mainly due to their nervous system.
Bats are said to possess acuity when it comes to their abilities to echolocate. Despite a noisy background, these animals are shown not to fail in catching insects and avoiding obstacles because of the presence of a double-channel pulsed Frequency-Modulation system (Kay), which is even considered to be more superior than human-made radars.
Nevertheless, perhaps the most unique characteristic of bats when it comes to their ability of echolocation is their silent behavior and cooperative sonar (Chiu Moss) that make them travel together and maintain their gregariousness. Bats are said to adjust their echolocation call frequencies to avoid signal jamming (Chiu Moss) from other bats. An individual bat is said to stop its echolocation in order to avoid signal interference from its fellows.
Shrews. Shrews are known to be the only terrestrial mammals that have been shown to echolocate specifically the two genera of North American shrews, Sorex and Blarina (Tomasi). The tenrecs which are native of Madagascar are said to be another species of shrews that possess echolocation. Tenrecs are insectivores of the family Tenrecidae whose tiny size ofeyes andnocturnal habits (Gould) are suggestive of the ability to echolocate.
Shrews are known to possess ultrasonic clicks, or self produced ultrasonic signals (Forsman Malmquist) as their distinct signal for echolocation in order to investigate the place they live, instead of catching prey just like bats do.
Dolphins. In contrast with the aerial bats and the terrestrial shrews, dolphins are aquatic cetaceans and are able to use echolocation in the deep waters. Dolphin echolocation, or also known as dolphin sonar, is used for navigation as well as for the accurate detection and localization of their prey. The mechanism is similar to that of bats and sperm whales, and consists of the same parts a transmitter, a receiver and a processor.
Dolphin sonar is said to outperform even man-made sonar systems (Dobbins) because it is believed to be able to work efficiently in shallow waters and can produce signals not disturbed by water turbulence, increased sound reverberations, and suspended sediment (Mo, Biologically Inspired). Dolphin sonar systems are accurate in that it can target animals like a small, 9-18 cm long sardine even at distances of zero to over 100 meters. (Mo, Biologically Inspired)
Dolphins in particular are equipped with two distinct types of sonar systems, or biosonar. There is the impulse-type, or click-type, biosonar (Fulton). This type of biosonar is mainly used for detecting targets precisely even within a range of 100 meters. The other type of dolphin biosonar is the swept continuous tone (whistle-based) (Fulton) biosonar which dolphins use to localize targets of about 600 meters with relatively less precision. Both types of dolphin biosonars work through the principle of the Doppler effect.
Whales. Just like dolphins, whales are aquatic mammals called cetaceans. Similarly, many species of whales use echolocation. Sperm whales, in particular, emit a series of clicks during their dives (Mo, How sperm whales). These clicks are said to increase their frequency as whales descend lower into the depths of the sea. Echolocation in whales is mainly used for obtaining precise information on the where a prey is located and in what direction it is moving.
Killer whales, on the other hand, are said to produce sounds for two functions that overlap communication and echolocation. A killer whale is said to produce clicks, receive the echo and then interpret it. Killer whales are also able to determine the size, shape, speed, distance, direction, and even some of the internal structure of objects in the water (Killer Whales). From this aforementioned information, one can therefore say that the killer whale has a much more distinct and precise echolocation system compared to other cetaceans.
However, a certain subspecies of killer whale, the transient killer whale, which are said to prey on a number of marine mammals, actually rely more on passive listening than on echolocation (Killer Whales) when they search for their prey. This is usually the case because marine mammals have excellent hearing and that even slight echolocation signals might drive them away from the transient killer whale.
Swiftlets. Perhaps the only species of birds known to possess echolocation are the cave-nesting swiftlets of the genus Aerodramus, which are native to Australia, Asia and the Pacific Islands. This type of bird nests in caves and produces clearly audible clicks while flying in dim light or complete darkness (Price). For these birds, their echolocation is primarily used for avoiding obstacles while they are in flight rather than for catching prey, which is characteristic of insectivorous bats.
The Evolution of Echolocation
Laryngeal echolocation in bats is known to have evolved from the so-called mammalian hearing gene known as Prestin (Teeling). This hearing gene encodes the protein sequences that make up the gene tree of the bottlenose dolphin, a toothed whale, and microbats (Ying, et al.). the gene may in fact have developed from how the first animals known to possess echolocation at present may have adapted themselves to their present environment. The hearing gene Prestin is said to be located in outer hair cells that serve as an amplifier in the inner ear, refining the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea. (In Bats and Whales)
Evolutionary biologist David Lindberg of the University of California-Berkeley co-authors a study on the evolution of echolocation in toothed whales and believes that just like bats which developed their ability of echolocation from chasing flying insects, the cetaceans developed sonar to chase squid at night (Whale and Dolphin). Lindberg adds that immediately after the first cetaceans moved to the ocean, they found this incredibly rich source of food surfacing around them every night and bumping into them. (Whale and Dolphin). Therefore, in order for them to adapt to this difficult yet essential lifestyle, the cetaceans developed echolocation.
The General Mechanism of Echolocation
Basic Mechanism of the Transmission of Sound Waves. With the latest technology, bats have been determined to produce echolocation through their larynx, thus their echolocation is called laryngeal echolocation. The technology used to determine this used detailed 3D scans of the internal anatomy of 26 different bats, representing 11 different evolutionary lineages (Bat researchers). This technique was employed by the Robarts Research Institute located in London, Ontario and this particular technique allowed the identification of a bone known as the stylohyal bone in the hyoid chain that connects the larynx to theeardrum in bats (Bat researchers). This led researchers to conclude that the larynx of bats is what they use to generate echolocation signals, which allows them to operate during the night.
The process of echolocation in bats is not as simple as the shrieking sound it emits during flight. From its larynx, it first emits a sonar sound that travels outwardto impinge on an insect at some target range (Simmons Galambos). The resulting reflections then return to the bat from various insect body parts (Simmons Galambos) such as its wings and head. Afterwards, the bat hears the transmitted sound directly in order to initiate processing of information. It does this by establishing a zero-time origin for reception of subsequent echoes (Simmons Galambos). This means that the most important part of the echolocation process is the time interval or delay that takes place right after the sound is transmitted back to the bat by the insect through an echo. The resulting echo will determine exact target range. The targets or sources of reflections such as insects are known as glints. (Simmons and Galambos)
For cetaceans, the entire process is slightly different. The first thing that happens is the production of bursts of clicks of varying frequenciesin a series of air sacsin the nasal cavities (Mo, Biologically Inspired). Second, the monkey lips-dorsal bursae complex (Au, et al. 367), open into the passage where the blowhole is. Third, the dolphins echolocation signals are emitted by an air-containing cavity into the water, which causes a certain mismatch in terms of the speed of the propagation of sound from the air to the water. Fourth, this imbalance is overcome by a structure called the melon, which is actually a large deposit of fatty issues located in the dolphins nasal sac muscles. The melon then slows down the sound waves as they are emitted into the water in order to produce a smoother sound propagation.
Characteristics of Echolocation Calls. Echolocation signals are characterized mainly by their intensity in terms of decibels, their frequency in kHz, and their duration in terms of milliseconds (ms). Echolocation calls of bats in particular are typically ultrasonic ranging in frequency from 20 to 200 kilohertz (kHz) (How Do Bats Echolocate), compared to human hearing at an average of 20 kHz. As to loudness, the echolocation calls of bats are as low as 50 dB and as high as 120 dB (How Do Bats Echolocate). This is said to be louder than the sound of a smoke detector placed 10 cm away from the ear. This is literally extremely loud. However since this sound is ultrasonic, humans are unable to hear it.
One particular characteristic of the echolocation signals of bats is that they are emitted with multiple harmonics and fairly broad frequency modulation (Simmons and Stein), which results in a very sharp resolution of the position of the target for the purpose of rejecting clutter interference. This type of signals is particularly used by bats in cluttered environments. This is the main reason why despite the noise of the surroundings, bats never lose track of any insect that it is destined to catch as prey.
For cetaceans, particularly dolphins, there are basically two kinds those capable of whistling and those that cannot whistle. Those capable of whistling include the bottlenose dolphin, or Tursiops sp., the Chinese river dolphin, or Lipotes vexillifer, and many others. The peak frequencies of these whistling cetaceans usually reach between 120,000 and 130,000 Hz. (Au et al. 365)
The second type of cetaceans includes those that are not known to emit any whistle signals. Their sounds are produces only in high frequency, low intensity click signals (Au et al.). The duration of these click signals is considered much longer and reach peak frequencies of up to 140,000 Hz. (Au et al. 368)
Frequency Range. Bats usually emit high pitched echolocation signals at usually a very high frequency in order to suit the type of prey that they will catch as well as their surrounding environment. The usual range is from 20 to 200 kilohertz (kHz) (How Do Bats Echolocate).
Echolocating large brown bats, or Eptesicus fuscus, usually have frequencies ranging from 110 kHz to 120 kHz and are believed to shift frequencies immediately to avoid broadcast-echo ambiguity in clutter (Hiryu et al.). This means that the frequencies of bat echolocation signals usually have a large range because they almost always shift frequencies depending on the amount of clutter in the surroundings so that the echoes transmitted back to them will remain clear.
For cetaceans, the echolocation signals may include the usual clicks, as well as whistles and chirps. The maximum frequency of a dolphin sonar in particular is estimated at 160,000 Hz (Mo, Biologically Inspired), while the lower limit is from 100 Hz to 8,000 Hz (Erber) and the upper limit is from 120,000 Hz to 200,000 Hz (Erber), and may considerably vary depending on the species, size, age and gender of the particular cetaceans. However, in dolphins, high frequency sonar between 1.8 and 2.4 MHz are ideal for providing a sufficient resolution for target recognition and identification (Wilcox Fletcher).
The Efficiency of the Echoes. The bat that is typically known to possess the ability for echolocation is the large brown bat, or the Eptesicus fuscus, which produces echoes of 11 to 12 dB, a delay of 2.3 to 4.6 ms at targets of 40 and 80 cm (Simmons et al.) respectively.
Cetaceans on the other hand can detect an object with the length of as small a value as 9-18 cm from a distance of 100 meters (Moh, Biologically Inspired).
This simply explains how amazingly precise the echolocation systems of cetaceans are.
Conclusion
Perhaps what makes echolocation an amazingly sophisticated physiological ability is the fact that it is exhibited by only a handful of animals bats, shrews, dolphins, whales and swiftlets. Echolocation mainly serves as a means of navigating in the dark, catching prey and for communication among members of the same species. Echolocation signals begin as sound waves formed in the larynx of bats and the nasal cavities of cetaceans. These sound waves then hit the target and are consequently transmitted back to the echolocating animal for split-second interpretation of the exact position and movement of the target. Moreover, the similarity in the evolutionary origin of the echolocating parts of bats, cetaceans and other animals further lead humans to keep asking more and more questions on the inherent abilities of these animals as well as on the significance of this amazing ability in both human psychology and technology.
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